Monday, October 31, 2011

The major energy sources in the United States

The major energy sources in the United States are petroleum (oil), natural gas, coal, nuclear, and renewable energy. The major users are residential and commercial buildings, industry, transportation, and electric power generators. The pattern of fuel use varies widely by sector. For example, oil provides 94% of the energy used for transportation, but only 1% of the energy used to generate electric power. Understanding the relationships between the different energy sources and their uses provides insights into many important energy issues.

Primary energy includes petroleum, natural gas, coal, nuclear fuel, and renewable energy. Electricity is a secondary energy source that is generated from these primary forms of energy.

Primary energy sources are commonly measured in different units: barrels (= 42 gallons) of oil, cubic feet of natural gas, tons of coal. To compare across fuels, we need to use a common unit of measure. The United States uses Btu, or British thermal units, which measure fuel use by the energy content of each fuel source.

Total U.S. energy use in 2010 was 98 quadrillion (=1015, or one thousand trillion) Btu. One quadrillion Btu, often referred to as a "quad," therefore represents about 1% of total U.S. energy use.

In physical energy terms, 1 quad represents 172 million barrels of oil (8 to 9 days of U.S. oil use), 50 million tons of coal (enough to generate about 3% of annual U.S. electricity use), or about 1 trillion cubic feet of natural gas (equal to 4% of annual U.S. natural gas use in 2010).

The number of quads used in 2010 from each primary energy source is shown in the pie chart on the left. Petroleum (oil) provides the largest share of U.S. primary energy, followed by natural gas, coal, nuclear energy, and renewable energy (including hydropower, solar, geothermal, wind, and biomass).

Primary energy is used in residential and commercial buildings (including homes, businesses, schools, and churches), in transportation, and by industry. Primary energy is also used to generate electricity. The bar chart shows the amount of primary energy used in each of these sectors. As you can see, electric power generation is the largest user of primary energy, followed by transportation.

The electric power sector uses primary energy to generate electricity, which makes electricity a secondary, rather than a primary, energy source. Nearly all electricity is then used in buildings and by industry. This means that the total levels of energy used by residential and commercial buildings, industry, and transportation are actually higher than the amounts shown on the graphics when electricity is added in.

The lines in the figure below connecting the primary-energy-sources on the left with the demand-sectors on the right summarize the source-sector linkages in the U.S. energy system. For example, because all nuclear energy is used in the electric power sector to generate electricity, and nuclear represents 21% of the primary energy used by that sector, the line between nuclear energy and the electric power sector shows 100% on the nuclear (supply source) side and 21% on the electric power (demand sector) side.

The mix of primary energy sources varies widely across demand sectors. Energy policies designed to influence the use of a particular primary fuel for environmental, economic, or energy security reasons often focus on sectors that are major users of that fuel.

For example, because 71% of petroleum (oil) is used in the transportation sector, where it provides 94% of the total energy used, policies to reduce oil consumption have tended to focus on the transportation sector. These policies usually seek to increase fuel efficiency or promote alternative fuels. Ninety-two percent of coal, but only 1% of oil, is used to generate electricity, suggesting that policies affecting electricity generation are likely to have a much larger impact on coal use than oil use.

Some primary energy sources, such as nuclear and coal, are entirely or predominately used in one sector. Others, like natural gas and renewables, are more evenly distributed across sectors. Similarly, while transportation is almost entirely dependent on one fuel (oil), electric power uses a variety of fuels.

The electric power sector has seen large changes in the fuel mix. A little over 50 years ago, nuclear energy played no role in electric power generation, but in 2010, nuclear energy provided 21% of the energy used to generate U.S. electricity. Oil provided a growing share of energy for electric generation through the 1960s, but its share has declined to 1% in 2010 since peaking at 18% in 1973.